Fluid flow restrictor having a variable coefficient of restriction

ABSTRACT

A valve for controlling fluid pressure and volume of flow in fluid systems utilizing a nozzle which issues a stream of fluid into a housing against the surface of an adjustable die. The length of gap separating the nozzle and die can be changed to effect a change in the coefficient of restriction of the valve without producing a significant change in the functional relationship between fluid volume flow and pressure drop therein. A substantially square root functional relationship is maintained by the valve over a range of gap lengths up to about 10 percent of the nozzle diameter.

United States Patent Eckerlin 1 June 27, 1972 [54] FLUID FLOW RESTRICTOR HAVING A VARIABLE COEFFICIENT OF RESTRICTION FOREIGN PATENTS OR APPLICATIONS 153,392 11/1920 GreatBritain ..138/44 683,456 11/1952 GreatBritain ..138/43 Primary Examiner-Herbert F. Ross Attorney-Clarence R. Patty, Jr. and Walter S. Zebrowski [5 7] ABSTRACT A valve for controlling fluid pressure and volume of flow in fluid systems utilizing a nozzle which issues a stream of fluid into a housing against the surface of an adjustable die. The length of gap separating the nozzle and die can be changed to effect a change in the coefficient of restriction of the valve without producing a significant change in the functional relationship between fluid volume flow and pressure drop therein. A substantially square root functional relationship is maintained by the valve over a range of gap lengths up to about 10 percent of the nozzle diameter.

4 Claims, 2 Drawing Figures FLUID FLOW RESTRICTOR IIAIIING A VARIABLE COEFFICIENT OF RESTRICTION BACKGROUND OF THE INVENTION In fluidic and fluid operated systems there is often a need for the accurate control of fluid flow rates and point-to-point pressure differentials. For example, in a cascade connected pair of proportional fluidic amplifiers, it is often necessary to adjust, downward, the level of output port pressure variations of the upstream stage to bring them within the operating range of the control ports of the succeeding stage. One way this has been accomplished in the prior art has been to connect a flow restriction device, called a restrictor, between an output port of the upstream stage and a control port of the succeeding stage.

The restrictor performs a function in fluid circuits which is somewhat analogous to the function of a resistor in an electrical circuit. Specifically, a restrictor establishes a functional relationship between volume or quantity of flow and pressure drop in the same sense that a resistor establishes a constant of proportionality, e.g., ohmic resistance, between current and voltage drop in an electrical circuit.

Here, however, the analogy ends. In a linear electrical net work, a resistance value can be varied using a potentiometer, variable resistor, or the like to obtain any desired constant of proportionality between current and voltage drop without destroying the directly proportional relationship therebetween, in accordance with Ohms law of resistance.

Unfortunately, due to the nature of fluid dynamics, there is no such fixed functional relationship analogous to Ohm s law with respect to fluid flow and pressure drop in conventional variable restriction devices. In the first place the relationship between quantity or volume of flow and pressure drop produced by a restriction may or may not be linear. More often the relationship is closer to a half power or square root relationship although other exponential relationships may exists as well. I

Nozzles and orifices are basic forms of fixed restrictors which are known to exhibit a square root relationship between volume of flow Q and pressure drop AP according to the expression,

Q I/K) AP', where K is the restriction coefficient. K is substantially constant for incompressible fluids, and with respect to compressible fluids, K is substantially constant when the changes in AP are not too great, i.e., not over about 5 psi. Nozzles and orifices have a fixed geometry and can readily be designed to provide a single desired value of restriction coefficient wherein the above-mentioned square root relationship holds.

In the second place is the case where restrictions have been provided with variable geometry so as to permit adjustment of the restriction coefficient thereof. The well known needle valve is an example of such a variable restrictor. However, adjustment of the needle setting not only affects'the restriction coefficient of the valve, but also affects the exponential relationship between the volume of fluid flow and pressure drop therein. The latter effect results from the fact that adjustment of the needle affects the flow geometry of the nozzle in the valve. A more accurate expression for the volume of flow-topressure drop relationship produced by prior art variable restrictors such as the needle valve is where h is the restriction coefficient for a given. needle setting, and x(h) is a variable quantity dependent, at least in part, upon the value of h. It will therefore be'apparent to those skilled in the art that such a valve is not capable of maintaining a fixed square root relationship for varying values of h. Indeed, in some instances it may not be possible to obtain such a square root relationship for any practical value of h. This is a most series disadvantage especially where proportional analog fluid systems are involved where it is most important to maintain a fixed functional relationship between Q and AI. It is therefore extremely difficult to predict and control lluid llow properties in circuits and systems having such valves.

SUMMARY OF THE INVENTION It is therefore an object of the instant invention to provide a variable restrictor wherein its restriction coefficient can be varied substantially without altering the functional relationship between the volume of flow and pressure drop therein.

It is a further object of the instant invention to provide a variable restrictor having a fixed square root functional relationship between the volume of flow and pressure drop therein.

It is yet another object of the instant invention to provide a variable restrictor having a fixed nozzle geometry.

Briefly, in accordance with the instant invention, a housing is provided together with a nozzle for issuing a stream of fluid into said housing. A surface means is disposed in the housing downstream of the nozzle with the gap between the nozzle and surface means being adjustable. An outlet passage is provided which communicates with the gap.

Additional objects, features, and advantages of the instant invention will become apparent to those skilled in the art from the following detailed description and attached drawing on which, by way of example, only the preferred embodiment of the instant invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a cross-sectional elevation view illustrating one embodiment of the instant invention.

FIG. 2 shows a cross-sectional elevation view illustrating another embodiment of the instant invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 there is shown a cylindrically shaped housing 10 containing a passage 12 therethrough which is of generally larger cross-section over end portions of its length than over a central portion thereof. The enlarged end portions of the passage 12 are threaded to receive conforming threads formed on the outside surface of an adjustable nozzle 14 and an adjustable member 16. The member 16 consists of a hollow adjustable control cylinder 18 which is threaded on the interior surface thereof in conformity with a movable threaded die 20 inserted therein, an unthreaded end portion 22 of which extends in close fitting relationship through a central constriction of the passage 12 and into the opposite end portion of the passage 12. The ends of the die 20 and the nozzle 14 form a gap 24 therebetween whose length can be selected by adjustment of the nozzle I4, the member 16, or both. The gap 24 communicates with an outlet port 26 through which a fluid flows, under pressure, after it has been emitted into the gap 24 by the nozzle 14 and interrupted by the end surface of the die 20.

In a specific example of the above-described invention, a variable restrictor was constructed of brass having a nozzle 14 whose orifice diameter was 0.04 inch. To provide sensitive control for small displacements of the die 20, the control cylinder 18 was constructed having a thread on the outside surface thereof of 32 threads/inch. The interior surface of the cylinder 18 was threaded in conformity with the threading on the die 20 and contained 40 threads/inch. In this manner, a vemier-Iike adjustment of the die 20 was made possible.

In actual experiments with the foregoing restrictor, it was found that a square root functional relationship between volume of flow therein and pressure drop between the nozzle 14 and outlet port 26 was substantially maintained for lengths of the gap 24 up to and including about 10 percent of the orifice diameter of the nozzle 14, or about 4 mils. However, even for lengths of the gap 24 up to 8 mils and beyond the departure from a pure square root relationship was only slight and could be considered essentially of that form for many types of applications where extreme accuracy is not required.

Over a range of lengths of the gap 24 extending from I up to 4 mils, the value of the coefficient of restriction K was variable from 1.35 down to 0.37 hr. (in, Hg)"lft., which dimension corresponds to volume of flow Q in cubic feet per hour and pressure differential AP in inches ofmercury for a square root restriction wherein,

Q (l/k)AP For a gap length of 8 mils the restriction coefficient was about 0.24 hr. (in. Hg)" /ft.

It should be noted that where the nozzle 14 is provided with an orifice of a different diameter from that of the above specific example, the length of the gap 24 could be varied over a different range of distances than those values previously given, without resulting in a significant departure from the square root functional relationship in the restrictor. Further, it is not essential that both the nozzle 14 and member 16 be adjustable although the ability to adjust both as desired provides additional tuning flexibility.

Referring now to FIG. 2 there is shown, in another embodiment of the instant invention, a housing 30 having a passage 32 therein which is threaded over a portion of its length, from the entrance thereof, in conformity with an adjustable threaded nozzle member 34. The member 34 is adapted to direct a fluid stream toward the rim of a disk shaped rotatable cam 36 which is disposed in an enclosed end portion 37 of the passage 32. A cam shaft 38 attached at one end to the cam 36 extends through the wall of the housing 30 in close fitting relationship to prevent the escape of fluid therethrough.

The end of the member 34 and the rim of the cam 36 form a gap 40 across which a fluid stream issuing from the member 34 is directed. The stream impinges upon a portion of the rim of the cam 36 and thereafter flows, under pressure, through the outlet port 42 which connects with external circuitry, not shown. By rotating the cam 36 or adjusting the member 34, or both, the coefficient of restriction of the restrictor can be altered. An advantage inherent in the instant embodiment is that the rate of change of the radius of the periphery of the cam 36 with respect to its angular displacement can be a logarithmic function, an exponential function, a linear function, or the like. For example, if the radius of the cam 36 varies as a logarithmic function of angular displacement about its axis of rotation, the length of the gap 40, and consequently, the restriction coefficient will vary correspondingly. Thus, such a variable restrictor has logarithmic tuning capability somewhat analogous to the well-known logarithmically tapered potentiometer used in the electrical arts. It should be noted that the end portion 37 should be large enough so that the cam 36 will rotate freely therein without contacting the interior surface of the housing 30 so as to cause binding and wearing of the cam 36.

Although the instant invention has been described with respect to specific details of certain embodiments thereof, it is not intended that such details be limitations on the instant invention except insofar as set forth in the following claims.

lclaim:

1. A variable restrictor comprising a housing defining a passage having a single entrance therein, the surface of said housing defining said passage being threaded from said entrance over at least a portion of the length thereof,

an adjustable nozzle member threaded at least part way into said passage for issuing a stream offluid into said housing,

a rotatable cam disposed in said housing downstream of said member in the path of said stream of fluid, said member and the peripheral surface of said cam opposite said member defining a gap, the length of said gap being variable at least in part by rotating said cam whereby a substantially square root functional relationship is maintained between the volume of fluid flow and pressure drop through said resistor, and

an outlet port through said housing communicating with said gap.

2. The restrictor of claim 1 wherein the length of said gap is variable up to and including about 10 percent of the orifice diameter of said member.

3.Avar iable restrictorcomprising a housing defining a passage avmg a single entrance therein, the surface of said housing defining said passage being threaded from said entrance over at least a portion of the length thereof,

an adjustable nozzle member threaded at least part way into said passage for issuing a stream offluid into said housing,

a rotatable cam disposed in said housing downstream of said member in the path of said stream of fluid, said member and the peripheral surface of said cam opposite said member defining a gap, the length of said gap being variable at least in part by rotating said cam, the peripheral surface of said cam radially varying with respect to the rotational displacement thereof in a logarithmic manner, and

an outlet port through said housing communicating with said gap.

4. A variable restrictor comprising a housing defining a passage having a single entrance therein, the surface of said housing defining said passage being threaded from said entrance over at least a portion of the length thereof,

an adjustable nozzle member threaded at least part way into said passage for issuing a stream of fluid into said housing,

a rotatable cam disposed in said housing downstream of said member in the path of said stream of fluid, said member and the peripheral surface of said cam opposite said member defining a gap, the length of said gap being variable at least in part by rotating said cam, the peripheral surface of said cam radially varying with respect to the rotational displacement thereof in a linear manner, and

an outlet port through said housing communicating with said gap.

i i i t 

1. A variable restrictor comprising a housing defining a passage having a single entrance therein, the surface of said housing defining said passage being threaded from said entrance over at least a portion of the length thereof, an adjustable nozzle member threaded at least part way into said passage for issuing a stream of fluid into said housing, a rotatable cam disposed in said housing downstream of said member in the path of said stream of fluid, said member and the peripheral surface of said cam opposite said member defining a gap, the length of said gap being variable at least in part by rotating said cam whereby a substantially square root functional relationship is maintained between the volume of fluid flow and pressure drop through said resistor, and an outlet port through said housing communicating with said gap.
 2. The restrictor of claim 1 wherein the length of said gap is variable up to and including about 10 percent of the orifice diameter of said member.
 3. A variable restrictor comprising a housing defining a passage having a single entrance therein, the surface of said housing defining said passage being threaded from said entrance over at least a portion of the length thereof, an adjustable nozzle member threaded at least part way into said passage for issuing a stream of fluid into said housing, a rotatable cam disposed in said housing downstream of said member in the path of said stream of fluid, said member and the peripheral surface of said cam opposite said member defining a gap, the length of said gap being variable at least in part by rotating said cam, the peripheral surface of said cam radially varying with respect to the rotational displacement thereof in a logarithmic manner, and an outlet port through said housing communicating with said gap.
 4. A variable restrictor comprising a housing defining a passage having a single entrance therein, the surface of said housing defining said passage being threaded from said entrance over at least a portion of the length thereof, an adjustable nozzle member threaded at least part way into said passage for issuing a stream of fluid into said housing, a rotatable cam disposed in said housing downstream of said member in the path of said stream of fluid, said member and the peripheral surface of said cam opposite said member defining a gap, the length of said gap being variable at least in part by rotating said cam, the peripheral surface of said cam radially varying with respect to the rotational displacement thereof in a linear manner, and an outlet port through said housing communicating with said gap. 